The medical community is constantly striving for less invasive techniques for treating patients. To this end, miniature x-ray emitters have been developed and have become an integral part of a variety of treatment protocols.
For example, in the case of coronary artery disease, balloon angioplasty or percutaneous transluminal angioplasty has become relatively common. In a large number of cases, however, restinosis occurs at the site where the angioplasty was administered. Restinosis is the re-narrowing or reclosing of the treated coronary artery and is largely related to the development of neo-intimal hyperplasia that occurs within an artery after it has been treated with a balloon or atherectomy device. In a sense, restinosis is scar tissue that forms in response to a mechanical intervention within a vascular structure. This scarring of the vessel can be so severe that blood flow through the vessel is obstructed. One known form of countermeasure involves the use of stents. A stent is a metal, generally tubular, vascular prosthesis which is implanted after angioplasty to mechanically hold the vessel lumen open. However, even with a stent in place, in-stent restinosis still tends to occur in a large percentage of cases.
To prevent or limit restinosis, drugs such as Heparin, Dexamethasome, Integralin, and others have been utilized. These drugs generally include anticoagulants and arterial smooth muscle proliferation inhibitors as well agents to prevent the aggregation of platelets.
It is also known that radiation is effective in reducing restinosis after angioplasty. In the past, radiation was administered by mounting a radioactive isotope on the tip of a catheter and inserting the catheter into a vessel until the emitter reaches a lesion location. The radioactive isotope would then emit gamma or beta radiation to treat the lesion. Unfortunately, the use of such radioactive isotopes requires protective shielding and special care in handling and disposal. Furthermore, such techniques result in irradiation throughout the length of the blood vessel although it is only necessary that a particular location be irradiated. Also, the depth of irradiation is difficult to control when utilizing such techniques thus presenting a further disadvantage.
More recently, x-ray devices have been developed which are capable of delivering radiation to remote locations in the body, including narrow passageways as small as blood vessels. Such devices include radiation emitters which can be switched on and off and do not require the use or handling of radioactive isotopes. For example, U.S. Pat. No. 5,428,658 entitled “X-ray Source With Flexible Probe” issued on Jun. 27, 1995, describes an x-ray emitter utilizing fiber optic cables. The fiber optic cables are designed to carry light waves which activate a photocathode in the emitter. The x-ray emitter is positioned at the distal end of a high voltage cable which is designed as an optical fiber 2-3 millimeters in diameter with a metal central core which carries the high voltage. The optical fiber is also utilized to transmit light pulses from a laser at the proximal end of the fiber to an electron emissive surface at the distal end. The electrons emitted from the surface generate an electric current in the high voltage gap of the x-ray unit resulting in x-ray emission.
A similar arrangement is shown and described in U.S. Pat. No. 6,319,188 entitled “Vascular X-Ray Probe” issued Nov. 20, 2001. In this case, an x-ray probe is formed at an optical fiber cable with a high voltage conductor embedded in the optical fiber. The optical fiber has an external ground coating and feeds power to a small x-ray emitter at the end of the cable. The optical fiber provides a conduit for optical irradiation, preferably in the form a laser beam, which is fed to a thermionic cathode mounted at the end of the light path. The laser beam heats the cathode causing it to emit electrons. An anode or target is positioned opposite the cathode within a vacuum chamber, and a ground lead is fed to the anode via an external coating over the emitter.
Devices of the type described above in the referenced patents have the ability to independently control both the operating voltage and the current thus allowing for independent control of depth of penetration of irradiation into tissue and the power generated by the source; however, the fact that in such devices the optical fiber for transmission of light radiation and the high voltage cable for the power supply are combined renders both suboptimal. The high voltage cable, which is made of a thick optical fiber, is generally not flexible enough to be applied as a catheter source similar to Iridium 192 source used in afterloaders for brachytherapy. The optical guide has high losses, and because light exits from the distal end of the fiber into the vacuum chamber in the area of a high electric field, it compromises the high voltage holdoff of the unit.
Therefore, it should be appreciated that it would be desirable to provide an x-ray emitter and apparatus that provides for the independent control of the operating voltage and current while at the same time offering the advantages of flexibility. A flexible catheter source of ionizing irradiation would be suitable for intraoperative radiation brachytherapy.